Filters for the filtration of blood are used in transfusion, extracorporeal circulation and dialysis to remove debris and microemboli from the blood. These filtrants can deliteriously affect these external systems by clogging and more critically compromising the safety of the patient if introduced into his blood. Since conventional screen filters (170 to 200.mu.) do not remove these aggregates of blood components, more elaborate filters have been developed to deal with this problem. Currently, there are three types of such filters in use; to wit, a Dacron wool filter (Swank); a polyurethane foam filter (Bentley); and a polyester mesh filter (Pall). The Swank and Bentley filters are both constructed in depth as to yield a large filtering and adhesive surface area comprised of interstices within the filter material (the size of which gradually decreases to less than 27.mu. for the polyurethane foam). The polyester mesh filter is a single-layer, woven screen of about 40.mu. pore size, pleated to obtain a large surface area.
Various studies have examined and evaluated one or more of these filters on the following bases: a) screen filtration pressure (SFP) post filter (the pressure required to force blood at a constant rate through a single filter with 20.mu. pores); b) pressure drop across filters; c) analysis of particle, leukocyte, platelet counts and blood chemistry before and after filtration; d) changes of oxygen partial pressure in patients with and without filtration; e) counting of microemboli by means of ultrasonic techniques and by utilizing a Coulter thrombocounter; and f) clinical studies of patient responses. Unfortunately, this data is not conclusive, nor were all tests run on each of the three filters. Certain of these tests suffer from intrinsic error and thus resulting experimental observations are not directly a manifestation of filter efficacy and can be misleading. For example, SFP is not only a function of microemboli buildup, but can also arise from thrombus formation on the screen itself. The Swank filter removes cellular microemboli effectively, but does not affect fat emboli which can account for 80% of the total emboli. Further associated problems with these prior art filters are: a) that they tend to clog with use, restricting flow and causing hemolysis; b) that they remove platelets and, therefore, alter blood coagulation-regulatory mechanisms; c) that changes in flow velocity can affect filtering action; and d) that the filters possibly introduce polymer fibers into the blood.
A filter for the filtration of blood should desirably exhibit certain properties. First, microemboli and foreign matter of all sizes must be effectively removed from the blood without changing the properties of the blood; i.e., removal of red blood cells, leukocytes and platelets or causing hemolysis. Secondly, the filter should have a low pressure drop which would remain constant as a function of time. Flow rates of up to 6 liters a minute must be at low pressure drops with effective filtration, and the filter must remain stable during pressure changes. Thirdly, a low-priming volume of blood or a means of removing entrapped air is required. Fourthly, the filter materials should not react deliteriously with the blood (i.e., cause thrombosis), nor should material, such as fibers, be introduced into the blood.